Pa-2 stimulation of murine lymphocytes causes the production of soluble lymphocyte stimulating factors (LSF). These LSF are produced by splenic T cells and thymus cells. Fractionation of thymocytes by peanut agglutinin into non-agglutinable cells (about 10%) and agglutinable cells (90%) followed by removal of the PNA and stimulation with Pa-2, showed that the non-agglutinable thymocytes were responsible for the production of LSF. These helper factors have been separated into 6 major pools by affinity chromatography. While each stimulates DNA synthesis and immunoglobulin production by spleen cells, five are mitogenic for thymocytes or T cells and the sixth is B cell specific. One of the earliest biochemical events following mitogenic lectin stimulation of lymphocytes is the activation of a membrane enzyme system which causes the methylation of phosphotidyl ethanolamine. A phosphocholine degrative pathway must also be activated in order for the cells to synthesize DNA or to produce soluble factors (LSF). The fluorescence activated cell surface has been used to identify lectins which differentially bind to subsets of lymphocytes. Several such lectins have been identified and are being used to fractionate lymphocyte subsets.